Another TSRGD / Traffic Signs Manual discrepancy :

TSRGD 2016 allows the supplementary plate to read simply “Hump” or “Humps” i.e. no arrow or distance.

However, Chapter 4 doesn’t offer this as an option. Instead, it says that the distance may only be omitted from plates that incorporate an arrow. This seems to be a hang over from the old TSRGD 2002.